Interferometric fiber optic gyroscopes ("IFOG") are well known in the art. Briefly, as is well known in the art, an IFOG is apparatus which utilizes a difference in phase of light that travels through a fiber optic coil, both clockwise and counterclockwise, relative to an axis of rotation of the fiber optic coil. In the art, such apparatus have been fabricated utilizing a superluminescent neodymium (Nd) doped glass fiber emitting substantially at a wavelength of 1.06 microns or a superluminescent erbium (Er) doped glass fiber emitting in a wavelength region substantially between 1.53 to 1.55 microns.
An article entitled "Progress in Interferometer and Resonator Fiber Optic Gyros" by G. A. Sanders, R. -Y. Liu and L. K. Strandjord, published in the Conference Proceedings of the 8.sup.th Optical Fiber Sensors Conference, Jan. 29-31, 1992, pp. 26-29 (article 1) and an article entitled "Integrated 1.06 um Fiber Gyro Superluminescent Source" by W. K. Burns, R. P. Moeller, E. Snitzer, and G. Puc, published in the Conference Proceedings of the 8.sup.th Optical Fiber Sensors Conference, Jan. 29-31, 1992, pp. 42-46 (article 2) disclose such prior art IFOG apparatus.
As is well known in the art, there are a number of ways in which an IFOG can be fabricated. As shown in FIG. 2 of article 1, the most common IFOG apparatus comprises a light source which is coupled into a lithium niobate (LiNbO.sub.3) integrated optic chip and a single mode, single polarization or polarization maintaining fiber coil. As further discussed in article 1, LiNbO.sub.3 integrated optics chips are commercially available for providing bias modulation and serrodyne modulation used in fabricating the IFOG apparatus. Further, the block diagram of FIG. 2 of article 1 provides an example of an IFOG apparatus which is fabricated from readily available circuits and devices which are well known to those of ordinary skill in the art.
As discussed in article 2, there is great interest in using fiber optic superluminescent sources in a fiber optic gyroscope as a replacement for semiconductor superluminescent diodes because of several potential benefits provided by such fiber sources. Such benefits are, for example, higher available power, better wavelength stability with temperature, and improved lifetime. As reported in article 2, such benefits would be expected when an IFOG apparatus is fabricated from an all-fiber source that is coupled into the gyro without bulk optic components. In fact, article 2 reports an all-fiber source wherein: (a) a samarium (Sm) doped fiber is butt coupled to a 0.5 W diode pump array at one end and is spliced to a Nd doped active fiber at the other end (the Sm doped fiber is substantially transparent at the 0.81 um pump wavelength and is heavily absorbing at 1.06 um, thereby providing isolation from the diode array facet) and (b) the Nd doped fiber is further spliced to an input lead of a 1.06 um isolator. As described in article 2, this arrangement advantageously avoids lasing due to feedback, i.e., the arrangement avoids backreflection of backward travelling superluminescent emission off a high reflectivity diode array facet. Further, such an all-fiber source can be spliced into the IFOG without using bulk optic components.
An important criterion used in fabricating an IFOG apparatus is that the light source emit over a broad wavelength interval with substantially no resonant structures within the interval, i.e., the source should appear similar to a natural light source. However, it is also useful to obtain as much light as possible in a single mode fiber in order to increase the sensitivity of the IFOG apparatus. To provide the required broad band, natural light source combined with high intensity, it has become accepted in the art to utilize a superluminescent light source.
In a superluminescent optical fiber light source, spontaneous emission is amplified by induced emission in propagating through a fiber that has been pumped to an inverted state and feedback that would generate laser oscillation is prevented from occurring. As a result, high intensity, amplified spontaneous emission is created.
Although Er doped glass fibers produce a superluminescent spectrum, the spectrum has an asymmetric shape and a small bandwidth.
In light of the above, there is a need in the art for a superluminescent light source which produces a superluminescent spectrum having a substantially symmetric shape, substantially no structure, and a relatively broad bandwidth. In particular, there is a need in the art for such a light source which can be fabricated as an optical fiber light source.